Laser marking of articles

Abstract

A method of marking or forming a transparent window in an article 6 , such as a security document, is provided in which areas of opacifying layers of printed ink 8, 10 on opposite surfaces of a polymeric film or substrate 12 are irradiated by laser radiation 4 of a selected wavelength. The opacifying layers 8, 10 have greater absorption characteristics for the laser radiation than the film or substrate 12 which is substantially transparent to the selected wavelength of the laser radiation so that after ablation of the area of printed matter 8 on one surface, the radiation passes through the substrate 12 substantially unaffected to ablate the area of printed matter 10 on the opposite surface. In the resultant article, the markings or windows formed by the areas of printing removed from each surface of the substrate 12 are in register, and provide a deterrent against counterfeiting.

What is claimed is:  
     
       1. A method of marking an article, the article comprising a substrate with opacifying layers on opposite surfaces of the substrate, said method comprising: 
       irradiating an area of the opacifying layer on one surface of the substrate with a single beam of laser radiation such that said area of the opacifying layer on said one surface is ablated by the laser radiation to produce a marking by removing said area of the opacifying layer on said one surface, wherein the single beam of laser radiation travels through the substrate and ablates an area of the opacifying layer on the opposite surface of the substrate to produce a marking by removing the area of the opacifying layer from the opposite surface of the substrate.  
     
     
       2. A method according to  claim 1  wherein the opacifying layers on opposite surfaces of the substrate have greater absorption characteristics for laser radiation than the substrate. 
     
     
       3. A method according to  claim 2  wherein the wavelength of the laser radiation is selected such that the opacifying layers absorb and are ablated by the radiation and the substrate is substantially unaffected by the laser radiation. 
     
     
       4. A method according to  claim 1  wherein the single beam of laser indication is a continuous beam directed at the area of opacifying layer on said one surface of the substrate. 
     
     
       5. A method according to  claim 1  wherein the single beam of laser radiation is a pulsed beam which is directed at the area of opacifying layer on said one surface of the substrate. 
     
     
       6. A method according to  claim 5  wherein a single pulse of laser radiation is directed at the area of opacifying layer on said one surface of the substrate, said single pulse having sufficient energy to ablate the printed matter on said one surface and then travel through the substrate to ablate the printed matter on the opposite surface of the substrate. 
     
     
       7. A method according to  claim 5  wherein a plurality of pulses of the single beam of laser radiation area directed at the area of opacifying layer on said one surface of the substrate. 
     
     
       8. A method according to  claim 1  wherein the substrate is formed of a transparent plastics material. 
     
     
       9. A method according to  claim 8  wherein the transparent plastics substrate comprises a film of transparent polymeric material. 
     
     
       10. A method according to  claim 8  wherein the substrate comprises a laminate of two or more layers of transparent polymeric material. 
     
     
       11. A method according to  claim 8  wherein the substrate is formed from any one or more of the following materials: polyethylene (PE); polypropylene (PP) or polyethyltetraphthalate (PET). 
     
     
       12. A method according to  claim 1  wherein the opacifying layers on the opposite sides of the substrate comprise printed matter. 
     
     
       13. A method according to  claim 12  wherein the opacifying layers of printed matter are formed from opacifying inks. 
     
     
       14. A method according to  claim 12  wherein the layers of opacifying ink comprise a pigment dispersed in a binder of cross-linkable polymeric material. 
     
     
       15. A method according to  claim 13  wherein the pigment comprises titanium dioxide pigment. 
     
     
       16. A method according to  claim 13  wherein the binder comprises a polyurethane based resin. 
     
     
       17. A method according to  claim 13  wherein the layers of opacifying ink include silica. 
     
     
       18. A method according to  claim 1  wherein the wavelength of the laser radiation falls substantially within the range from 0.5 microns to 20 microns. 
     
     
       19. A method according to  claim 18  wherein the wavelength of the laser radiation is approximately 1.06 microns. 
     
     
       20. A method according to  claim 18  wherein the wavelength of the laser radiation is approximately 10.6 microns. 
     
     
       21. A method of creating a transparent window in an article comprising a clear substrate with opacifying layers on opposite surface of the substrate, said method comprising: 
       irradiating an area of the opacifying layer on one surface of the substrate with a single beam of laser radiation such that said area is ablated by the laser radiation to remove said area of the opacifying layer from said one surface, wherein the single beam of laser radiation travels through the substrate and ablates an area of the opacifying layer on the opposite surface of the substrate to create a transparent window in the substrate.  
     
     
       22. An article comprising a substrate having opacifying layers on opposite surfaces of the substrate, the substrate being formed from a material which is substantially transparent to a single beam of laser radiation of a selected wavelength, wherein each of said surfaces has a marking formed therein, said markings being formed by removing a first area of the opacifying layer on one surface of the substrate by ablating said first area with a single beam of laser radiation of the selected wavelength, and allowing the single beam of laser radiation to pass through the substrate to remove an area of the opacifying layer on the other surface in register with said first area removed from the opacifying layer on said one surface. 
     
     
       23. An article according to  claim 22  wherein the opacifying layers on opposite surfaces of the substrate have greater absorption characteristics for laser radiation than the substrate. 
     
     
       24. An article according to  claim 22  wherein the substrate is formed of a transparent plastics material. 
     
     
       25. An article according to  claim 24  wherein the transparent plastics substrate comprises a film of transparent polymeric material. 
     
     
       26. An article according to  claim 24  wherein the substrate comprises a laminate of two or more layers of transparent polymeric material. 
     
     
       27. An article according to  claim 22  wherein the opacifying layers on the opposite sides of the substrate comprise printed matter. 
     
     
       28. An article according to  claim 22  wherein the article is a security document. 
     
     
       29. An article or security document according to  claim 22  wherein the substrate is formed from any one or more of the following materials: polyethylene (PE); polypropylene (PP) or polyethyltetraphthalate (PET). 
     
     
       30. An article according to  claim 22  wherein the substrate is substantially transparent to laser radiation having a wavelength falling substantially within the range from 0.5 microns to 20 microns. 
     
     
       31. An article according to  claim 22  wherein the substrate is substantially transparent to laser radiation having a wavelength of approximately 1.06 microns. 
     
     
       32. An article according to  claim 22  wherein the substrate is substantially transparent to laser radiation having a wavelength of approximately 10.6 microns. 
     
     
       33. A security document comprising a clear substrate formed of a transparent plastics film having opacifying layers of printed matter on opposite surfaces of the film, the substrate being formed from a material which is substantially transparent to a single beam of laser radiation of a selected wavelength wherein both of said surfaces have a marking or a transparent window, said markings or windows being formed in the printed matter by removing a first area of the printed matter on one surface of the substrate by ablating said first area with a single beam of laser radiation of the selected wavelength, and allowing the single beam of laser radiation to pass through the substrate to remove a second area of the printed matter on the other surface, said second area being in register with said first area removed from the printed matter on said one surface. 
     
     
       34. A security document according to  claim 33  wherein the opacifying layers of printed matter are formed from opacifying inks. 
     
     
       35. A security document according to  claim 34  wherein the layers of opacifying ink comprise a pigment dispersed in a binder of cross-linkable polymeric material. 
     
     
       36. A security document according to  claim 35  wherein the pigment comprises titanium dioxide pigment. 
     
     
       37. A security document according to  claim 35  wherein the binder comprises a polyurethane based resin. 
     
     
       38. A security document according to  claim 35  wherein the layers of opacifying ink include silica.